When conformal coating splits, look for process residues.

Conformal coating cracking (Figure 1) was found after an assembly was returned from the field for investigation. The assembly was for a medical application produced in medium volume using a SnPb assembly process.

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It was determined some form of contamination under the mask had led to cracking on the coating. There was no evidence of corrosion or electrical failure on the sample board assembly. Large amounts of no-clean flux have been seen to cause lifting, but not corrosion on boards. This is probably caused by expansion and contraction of the material during thermal cycling. This movement of the material can lead to cracking of some brittle masks.

To properly diagnose the cracking, it would be necessary to review the board cleanliness and coating compatibility with the intended application. Temperature cycling is probably the cause for the cracking, but something under the mask caused it to expand and contract. If a mask is cracked, it cannot provide suitable protection to the surface of the board. If a no-clean flux is the root cause, the volume of material left on the board surface during assembly needs to be reviewed.

These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (defectsdatabase.npl.co.uk), available to all Circuits Assembly readers, allows engineers to search and view countless defects and solutions, or submit defects online.

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .